16.1 c) REACTION MECHANISMS
*Quite often, chemical changes are too complicated to happen in one simple stage. Instead, the
reaction may involve a series of small changes one after the other.
*A ______________ ______________ is a sequence of elementary steps that make up an overall
reaction.
*Each step, called an ________________ reaction (or elementary step), involves one-, two-, or
three-particle collisions.
Example:
Consider the following overall reaction:
2NO(g) + O2(g)  2NO2(g)
Proposed two-step mechanism: (Each step is called an ________________ reaction.)
Step 1:
Step 2:
NO(g) + O2(g)  NO3(g)
NO3(g) + NO(g)  2NO2(g)
*The sum of the elementary reactions yields the overall reaction. Notice that NO3(g) is produced in the first step and
consumed in the second step. NO3(g) is an example of a reaction ________________.
*Reaction intermediate  a chemical entity formed in an elementary reaction and ____________ in a subsequent
elementary reaction (Reaction intermediates do not appear in the _____________ reaction; they are finite, measurable.)
The Molecularity of Elementary Reactions
*Molecularity – refers to the __________ of reactant particles that are involved in an elementary reaction.
*Unimolecular elementary reaction  occurs when ______ molecule or ion reacts
e.g. Cl2(g)  2Cl(g)
e.g. O3(g)  O2(g) + O(g)
*Bimolecular elementary reaction  occurs when _______ particles collide and react
e.g. O3(g) + O(g)  2O2(g)
e.g. H2(g) + I2(g)  2HI(g)
*Termolecular elementary reaction  involves __________ particles colliding all at once(very rare)Why?
Practice Problems:
1. The following two-step mechanism has been proposed for the gas-phase decomposition of nitrous oxide (N2O)
Step 1: N2O(g)  N2(g) + O(g)
Step 2: N2O(g) + O(g)  N2(g) + O2(g)
(a) Determine the overall equation. 
(b) Identify the reaction intermediate(s). 
(c) What is the molecularity of each elementary step? Step 1____________ ; Step 2 ____________
Rate Law Equations for Elementary Reactions
*The rate law for an elementary step follows directly from the molecularity of that step. (Exception to follow.)
That is,
 unimolecular E.R’s are always _________ order
 bimolecular E.R’s are always _________ order
 termolecular E.R’s are always __________ order
*Mechanisms are _________________ rather than definitively stated. Proposed mechanisms must satisfy the following
criteria:
1.
2.
3.
The equations for the elementary steps must combine to give the equation for the _________ reaction.
The proposed mechanism must be reasonable.
The mechanism must support the _____________________ determined _________ _________.
The Rate-Determining Step (RDS) and the Rate Law:
*Elementary reactions in mechanisms all have different rates. The ________________ elementary reaction, the
___________-determining step, determines the overall rate.
Practice Problem:
Propose the rate law for the following reaction: 2NO2(g) + Cl2(g)  2NO2Cl(g)
Proposed mechanism:
Step 1:
NO2(g) + Cl2(g)  NO2Cl(g) + Cl(g) (slow)
Step 2:
NO2(g) + Cl(g)  NO2Cl(g)
(fast)
*Rate-determining step = Step ________
*Rate law for Step 1 = proposed rate law for the overall reaction; that is: Rate1 = ________________
*The proposed rate law must be consistent with the ________________ determined rate law in order for the proposed
mechanism to be considered reasonable.
More on Mechanisms:
Some possible mechanisms for the reaction: 2A + B ==> C + D
*RDS = rate-determining step
Mechanism
I
*II
*III
IV
V
A + B ==> X + C
A + X ==> D
A + B ==> X
A + X ==> C + D
A + A ==> A2
A2 + B ==> C + D
A + A ==> A2
A2 + B ==> C + D
B ==> X
X + A ==> Y + C
Y + A ==> D
Rate Expression
Slow RDS
Fast
Fast
Slow RDS
Fast
Slow RDS
Slow RDS
Fast
Slow RDS
Fast
Fast
Rate

Rate

Rate

Rate

Rate

Summary:
I
 Rate depends on the rate of collisions between _______ and ______.
Rate Law: Rate
 [A] [B]
*II  Rate depends on the rate of collisions between A and X; however, the formation of
X depends on the rate of collisions between _____ and ______.
Therefore:
Interpreted:
A + X ==> C + D
A + (A + B) ==> C + D
Rate Law: Rate
 [A]2[B]
Slow RDS
*III  Rate depends on the rate of collisions between A2 and B; however, the formation
of A2 depends on the rate of collisions between _____ and _____.
Therefore:
A2 + B ==> C + D
Slow RDS
Interpreted:
(A + A) + B ==> C + D
Rate Law: Rate
 [A]2[B]
IV  Rate depends on the rate of collisions between ___and ___.
Rate Law: Rate
 [A]2
V  Rate depends on the rate of collisions between B and the container walls.(…or
rate depends on the rate of decomposition of B.)
Rate Law: Rate
 [B]